B767 FLC FAILED TO MEET XING RESTR DUE TO A PROGRAMMING ERROR.

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THE FO WAS FLYING AND DOING HIS OWN CPU/FMC WORK. HE IS VERY PROFICIENT IN THE USE OF THE FMS; AFDS; ETC. HE HAS SEVERAL THOUSAND HRS IN THE B767-200. BTWN US WE HAD FLOWN THIS PARTICULAR ARR INTO PHL 300-325 TIMES. WE WERE AT FL280 AWAITING FURTHER STEPDOWN. A FAIRLY STANDARD CLRNC WAS GIVEN TO BE AT FL200 35 MI NE OF THE MANTA INTXN. WE WERE ON A DIRECT COURSE TO MANTA. THE FO INFORMED ME THAT HE HAD BUILT IN AN ADDITIONAL BUFFER OF 10 MI TO 45 TOTAL MI FOR THE DSCNT. I GLANCED AT THE PROGRESS PAGE AND SAW THE 10 MI SPREAD BTWN THE 'MAN02' WHICH WAS THE WAYPOINT HE HAD BUILT AND THE NEXT FIX. I DID NOT CHK FURTHER. THE 10 OR A 5 MI BUFFER HAS ALMOST BECOME STANDARD PROC BECAUSE THE ACFT HAS A TENDENCY NOT TO MAKE THE SELECTED ALT RESTR. SHORTLY THEREAFTER THE FO GOT OUT OF HIS SEAT TO GO TO THE LAVATORY. HE INFORMED ME THE ACFT WOULD BE STARTING DOWN SHORTLY. I GLANCED DOWN AND SAW TO TOP OF DSCNT AT 10 MI. ALMOST SIMULTANEOUSLY A CHANGE TO ZNY WAS GIVEN. I CHKED IN WITH 'FL280 CLRED TO FL200.' THE CTR IMMEDIATELY ASKED IF WE WERE IN A DSCNT AND I STATED WE WOULD BE STARTING DOWN IN ABOUT 10 MI. THE CTR REPLIED THAT WE WERE 43 MI FROM MANTA (8 MI FROM THE RESTR). I IMMEDIATELY BEGAN AN IDLE DSCNT WITH SPD BRAKES EXTENDED. THE FO TURNED BACK AND CLBED INTO HIS SEAT STATING WE HAD PLENTY OF ROOM. THERE WAS NO TIME TO DISCUSS THE SIT AND I CONTINUED THE DSCNT. NY THEN ADVISED US NO PROB; A NORMAL DSCNT WOULD BE OK. WE CROSSED MANTA AT APPROX FL210. IT WAS DIFFICULT TO FULLY ASSESS WHAT HAD OCCURRED UNTIL AFTER ARR AT PHL. THERE I REALIZED THE SYS HAD BEEN PROGRAMMED TO ARRIVE AT FL200 10 MI FROM MANTA. THE ORIGINAL 35 MI FIX HAD NOT BEEN ENTERED WHICH WOULD HAVE GIVEN A TOTAL OF 45 MI. CONCLUSIONS: I DID NOT CHK THE PROGRAMMING COMPLETELY. I RELIED COMPLETELY ON THE ABILITY OF SOMEONE I KNEW AND HAD WORKED WITH MANY TIMES PREVIOUSLY. I NOW THINK IT IS IMPERATIVE WHEN ANY ENTRIES OF ANY SORT ARE MADE THERE SHOULD BE NO EXCEPTIONS TO DOUBLE-CHKING OF THOSE ENTRIES BY THE OTHER PLT OR PLTS.